Catalytic Activity in Lithium-Treated Core−Shell MoOx/MoS2NanowiresDustin R. Cummins,†,‡ Ulises Martinez,† Rajesh Kappera,†,§ Damien Voiry,§ Alejandro Martinez-Garcia,‡

Jacek Jasinski,‡ Dan Kelly,∥ Manish Chhowalla,§ Aditya D. Mohite,† Mahendra K. Sunkara,*,‡

and Gautam Gupta*,†

†MPA-11, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States‡Department of Chemical Engineering and Conn Center for Renewable Energy, University of Louisville, Louisville, Kentucky 40292,United States§Materials Science and Engineering, Rutgers University, Piscataway, New Jersey 08854, United States∥Chemistry Division, Chemical Diagnostics and Engineering Group, Los Alamos National Laboratory, Los Alamos, New Mexico87545, United States

*S Supporting Information

ABSTRACT: Significant interest has grown in the development of earth-abundant and efficientcatalytic materials for hydrogen generation. Layered transition metal dichalcogenides presentopportunities for efficient electrocatalytic systems. Here, we report the modification of 1D MoOx/MoS2 core−shell nanostructures by lithium intercalation and the corresponding changes inmorphology, structure, and mechanism of H2 evolution. The 1D nanowires exhibit significantimprovement in H2 evolution properties after lithiation, reducing the hydrogen evolution reaction(HER) onset potential by ∼50 mV and increasing the generated current density by ∼600%. Thehigh electrochemical activity in the nanowires results from disruption of MoS2 layers in the outershell, leading to increased activity and concentration of defect sites. This is in contrast to the typicalmechanism of improved catalysis following lithium exfoliation, i.e., crystal phase transformation.These structural changes are verified by a combination of Raman and X-ray photoelectronspectroscopy (XPS).

1. INTRODUCTION

Hydrogen offers the potential for clean energy, but improve-ments in efficient and sustainable production of hydrogen fuelsare required to realize its potential. Currently, 90% of hydrogenis produced from fossil resources via thermal steam reforming(coal and natural gas).1−3 While alternative technologies suchas water electrolysis4 and photolytic water splitting5 are beingpursued, most of these technologies require expensive platinumand platinum-based composites,6−9 which can be readily“poisoned” by chemical contaminants. Transition metaldichalcogenides (TMDs) as catalysts for the hydrogenevolution reaction (HER) provide an opportunity to addressthese issues.10−31

TMDs are earth-abundant materials with the chemicalformula MX2, in which M represents a transition metal fromgroup IV, V, or VI, and X corresponds to a chalcogen (S, Se, orTe). In particular, MoS2 has been demonstrated to be apotentially efficient catalyst for HER.20 Bulk MoS2 has ahexagonal (2H-MoS2) crystal structure with its edges beingcatalytically active, while the basal plane is largely inac-tive.18,19,29,32 HER properties of several MoS2 morphologieshave been studied, including particles,16,30 nanowires,17 doublegyroid structures,19 CVD grown crystals,18 vertically orientedsheets,24−26 chemically exfoliated single layers,14,27 joining with

other TMD cocatalysts,33 and combination with carbonstructures to improve conductivity.22,23

For model MoS2 structures, HER activity has been attributedto either edge sites and/or phase transformation of the basalplane.14,18 Extensive analysis by Chianelli et al. has demon-strated that 2H-MoS2 has a trigonal prismatic orientation, witheach sulfur atom strongly bound to three Mo atoms to form thebasal plane, making this surface chemically inert.34 Any catalyticprocesses occur at sulfur vacancies, which are moreconcentrated at the edge of the crystal. These edge siteswhere the S−Mo dimer and S defects occur more often, havemore conducting, metallic properties, compared to the bulkcrystal,35 and are the preferential sites for adsorption andcatalysis.18,36,37

A metastable phase of MoS2 with a trigonal crystal structureand octahedral orientation (1T-MoS2) has been shown to havesignificantly different electronic properties than the hexagonal2H phase. 2H-MoS2 has an indirect band gap of 1.2 eV and hassemiconductor properties.38 The bulk semiconductor can bechemically39 or mechanically exfoliated;40 single layers of MoS2

Received: June 12, 2015Revised: September 19, 2015Published: September 22, 2015

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© 2015 American Chemical Society 22908 DOI: 10.1021/acs.jpcc.5b05640J. Phys. Chem. C 2015, 119, 22908−22914

exhibit a direct band gap transition of 1.9 eV. The hexagonal totrigonal phase transformation is typically achieved byintercalation of lithium between the layers, which donates anelectron and creates strain in the crystal lattice.41−44 This 1T-MoS2 is metallic in nature13,45 and has high catalyticperformance for HER.14,26−28

In this paper, we use lithium intercalation to modify 1Dcore−shell MoOx/MoS2 nanowires, which results in an efficientHER catalyst. Core−shell MoOx/MoS2 nanowires have beenreported previously and possess good electrocatalytic activity;however, the MoS2 shell is composed of the 2H semi-conducting phase, and the basal planes are oriented parallel tothe nanowire axis, possessing a lower number of active sites.17,20

Our results indicate that lithium intercalation disrupts the MoS2shell and creates numerous edge defects while maintaining thenanowire 1D morphology, which leads to increased hydrogenevolution characteristics. This destruction of ordered crystalstructures has been well studied in lithium-ion battery electrodeapplications,46 and dramatic volume expansions have beenshown during lithium intercalation of graphene, ranging fromonly 10% for elemental lithium to a few hundred percentincrease when using lithium solvents.47

2. MATERIALS AND METHODS

2.1. Synthesis of Nanostructured Materials. Thesynthesis of MoO3 nanowires is described in previouswork.17,48 The sulfurization of these oxide nanowires wasperformed at 300 °C for 2 h in a 15 Torr 99% H2S atmosphere.Lithium exfoliation of these nanowires, as well as the formationof the 2D sheets, was performed by soaking in n-butyl lithiumfor 24 h, then heavy rinsing with DI water to fully remove thelithium. This is shown schematically in Figure S1.

2.2. Electrochemical Characterization. The rinsed nano-wires are dispersed in DI water and then transferred to a glassycarbon electrode. Further, a thin layer (∼2.7 μL/cm2) of 5%Nafion solution is deposited on top of the electrode to preventthe nanowires or sheets from delaminating and to enhanceproton conductivity. The HER activity is measured in 0.5 MH2SO4 (pH = 0) electrolyte in a three-electrode configurationwith Ag/AgCl reference electrode (+0.210 V vs RHE), graphiterod as counter electrode, and glassy carbon with or withoutMoS2 nanostructures as working electrode. The electrolyte ispurged with nitrogen to remove oxygen from the system. Theworking electrodes are subjected to initial cycling (−0.35 to+0.2 V vs RHE) to remove surface contamination and are

Figure 1. Micrographs of MoS2 nanowires and sheets before and after lithiation. (A) SEM image of the as-grown nanowires. (B) TEM of the as-grown nanowires, showing the ordered MoS2 shell on the MoOx core. (C) SEM of the MoS2 nanowires after lithiation. (D) TEM of the nanowiresafter lithiation, showing the disordered crystal shell. (E) SEM of bulk 2H-MoS2 powder. (F) SEM of the lithium exfoliated MoS2 sheets. (G) STEMof the 1T flakes after lithiation, with an inset showing a molecular model of 1T-MoS2. The 1T phase can be distinguished by STEM due to a changein contrast from the sulfur atoms. In the 2H-MoS2 lattice, two sulfur atoms overlap when viewed in profile; however, in the 1T phase, due to thelattice rotation, there is only one sulfur atom, which leads to the intensity of the imaged sulfur in 2H to be twice as bright. This is explained morefully in multiple fundamental studies.13,15,44

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cycled until a steady state is achieved prior to collection of thedata. In an attempt to standardize the electrocatalyticperformance, every effort is taken to ensure a similar loadingof catalytic material on the electrode, both nanowires and 2Dsheets.Since the metastable 1T-MoS2 structure is notoriously

sensitive to slightly elevated temperatures,45 the active materialis heated in an inert atmosphere at various temperatures for 1 hand then allowed to cool under vacuum. The annealed materialis then transferred to the glassy carbon electrode, and theelectrochemical testing is performed as described previously.

3. RESULTS AND DISCUSSION3.1. Effect of Lithiation on Nanowires and 2D Sheets.

MoO3 nanowire arrays were grown on stainless steel substratesusing hot filament chemical vapor deposition (HFCVD) andexposed to H2S

17,48 (15 Torr 99% H2S at 300 °C). Figure 1shows electron microscopy images of MoS2 nanostructuresbefore and after lithiation. Nanowires form a vertically orientedarray, with diameters ranging from 20 to 50 nm and lengths of1 to 3 μm, as seen in Figure 1A. HR-TEM imaging (Figure 1B)shows the oriented crystalline 2H-MoS2 shell with a thicknessof 5−10 nm around a single-crystal MoOx core. TEMmeasurements indicate that the MoS2 shell is single crystallineacross the radius of the shell but has low angle grain boundariesalong the length of the nanowire, with the basal plane of 2H-MoS2 (100) oriented parallel to the length of the nanowire.Chemical exfoliation is performed by reacting the as-synthesized nanowires in n-butyl lithium solution for a desiredtime, followed by thoroughly rinsing with DI water to removeexcess lithium. The lithiation and subsequent washing stepsslightly disrupt and delaminate the nanowire array as a whole,which is evidenced in the SEM image (Figure 1C), but theindividual nanowire morphology is unchanged. Upon exami-nation with HR-TEM, it can be readily observed that thelithium intercalation (Figure 1D) significantly disrupts theordered crystal structure of the MoS2 shell, leading to apolycrystalline MoS2 shell with random orientations andincreased edge defects while still maintaining the nanowiremorphology. Electron energy loss spectroscopy (EELS)analysis is used to demonstrate that there is almost no lithiumresidue in the MoS2 nanowires after extensive washing

15 and isreported in the Supporting Information (Figure S3). A similarprocedure is used for lithiation of bulk MoS2 powder (Figure

1E) to form single-layer 1T-MoS2 sheets (Figure 1F). STEManalysis of the 2D sheets (Figure 1G) shows that each sheet issingle crystalline with predominant phase to be 1T-MoS2.

3.2. Electrochemical Activity of Catalysts for Evolu-tion of H2. Electrochemical HER studies were performed toinvestigate the effect of lithiation. The catalytic materials weredeposited (approximately 0.1 mg) on glassy carbon electrodes,and electrochemical measurements were performed in 0.5 MH2SO4. All data are iR corrected to account for resistanceinherent in the testing system (∼12 Ω); the Nyquist plotshowing the series resistance is reported in the SupportingInformation (Figure S4).A 50 mV shift (from −0.2 to −0.15 V vs RHE) in the onset

potential (Eonset) was obtained for the nanowires followingchemical intercalation, as seen in Figure 2A. Moreover, asignificant 6-fold increase in current density was observed, from∼4 to ∼25 mA/cm2 at −0.35 V. These results clearly indicate asignificant enhancement in electrochemical activity uponchemical exfoliation. Bulk MoS2 powder is also analyzed forcatalytic activity in a similar manner. It shows an HER onsetpotential of ∼−0.8 V and a large Tafel slope of 156 mV/decadeindicating extremely poor catalytic activity (Figure S5). Forcomparison, we also include the results from chemicallyexfoliated nanosheets forming the metallic 1T-MoS2 (lithiated)sheets that show high HER activity due to phase trans-formation14 with an onset potential of −0.15 V and Tafel slopeof 52 mV/decade as seen in Figure 2A and B. Tafel slopes of allthe catalysts are reported in Figure 2B to identify themechanism of hydrogen evolution.

3.3. Reaction Pathways for Hydrogen Evolution. Inorder to determine the mechanism of hydrogen evolution at theelectrode interface, detailed analysis of Tafel plots is performedto indicate the rate-limiting step. Typically, there are threeelementary rate-determining reactions involved in HER at anycatalyst surface. Specifically, the proton adsorption (known asthe Volmer (eq 1) step), followed by either (a) the evolution ofmolecular hydrogen by the combination of an adsorbed protonand a proton from solution (Heyrovsky (eq 2) step) or (b) thecombination of two adsorbed protons (Tafel (eq 3) step).49,50

Two main pathways for hydrogen evolution are Volmer−Heyrovsky (1−2) or Volmer−Tafel (1−3).

+ →+ −Volmer: H e Hads (1)

Figure 2. (A) Linear sweep voltammograms showing the HER catalytic activity of MoS2 nanowires, both as synthesized (red curve) and afterlithiation (black curve). The HER activity for chemically exfoliated 1T-MoS2 (green curve) 2D sheets is included for comparison. (B) Tafel slopes ofMoS2 nanostructures. The HER activity for bulk MoS2 powder (blue dotted curve) is shown in the Supporting Information, as it is outside the rangeshown.

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+ + →+ −Heyrovsky: H H e Hads 2 (2)

+ →Tafel: H H Hads ads 2 (3)

Experimental observation and kinetic modeling show that if theadsorption of a proton, i.e., the Volmer step, is the rate-determining step then a Tafel slope of ∼120 mV/decade shouldbe observed. However, if the evolution of molecular hydrogenis the rate-determining step, then a Tafel slope of ∼40 mV/decade or ∼30 mV/decade should be observed, indicative ofthe Heyrovsky or Tafel steps, respectively.8,49,50 It is importantto understand that these values will only hold if the same rate-determining step is homogeneous throughout the catalyst.However, this is rarely the case. Multiple reactions occur at anygiven applied potential as a consequence of different reactionsites. Analysis of the change in Tafel slopes of the MoS2nanowires before and after lithiation (∼90 and ∼50 mV/decade) shows a change in the rate-determining step, shiftingfrom adsorption limited to hydrogen evolution limited,suggesting the exposure of more adsorption sites (increase inactive defect sites vs relatively inactive basal plane).As shown in Figure 1B, as-grown MoS2 nanowires have a 5−

10 nm highly oriented, single crystalline shell on the reducedoxide core. The relatively inert basal plane of 2H-MoS2 isparallel to the nanowire length, so there are few edge sitesexposed for hydrogen adsorption. After lithiation, a polycrystal-line shell with random orientations and a higher concentrationof more active defect sites is obtained as shown earlier (Figure1D). Measurment of the double-layer capacitance (Figure S6)via cyclic voltammetry at higher scan rates51 shows that theactual, wetted surface area of the nanowires does not changesignificantly following the lithium exfoliation; the increasedcatalysis is due predominantly to the exposure of active sites.These phenomena, i.e., increased exposure of defect sites forabsorption and a conductive, high surface area architecture,explain the significantly improved onset potential and currentdensity of the HER on exfoliated MoS2 nanowires. Bulk MoS2powder has a Tafel slope of 156 mV/decade, and after lithiumexfoliation, a 56 mV/decade Tafel slope is obtained for the 2Dsheets; this significant improvement in kinetics is attributed tophase transformation.14,26

3.4. Determination of Phase and Binding Energies ofSynthesized Nanostructures. Although TEM imagingindicates different origins of catalytic behavior between thecore−shell MoOx/MoS2 nanowires and the 2D exfoliated

sheets, the exfoliated nanowires show similar Tafel slopes andcatalytic performance, so further correlated Raman and XPSmeasurements are performed to understand the real origin ofthe catalysis in the nanowires.Raman spectroscopy (Figure 3A) is used to confirm the

presence of the trigonal and octahedral phases. 2H-MoS2 hastwo strong Raman peaks at ∼382 and ∼407 cm−1,corresponding to the E1

2g and A1g, respectively, as well as abroad peak at 454 cm−1, which corresponds to two peaks, asecond-order zone-edge phonon peak 2LA(M) and a first-orderoptical phonon peak, A2u.

52,53 The 1T phase has unique Ramanvibrational peaks at ∼220 (J2) and ∼325 cm−1 (J3)

45 which arehighlighted by the shaded area in Figure 3A and are clearlyevident in the chemically exfoliated MoS2 sheets (green curve).These vibrational modes are only active when the 1T phase ispresent. In comparison, Raman analysis of the MoS2 nanowirearrays before (black curve) and after lithiation (red curve)shows no evidence of any phase transformation from the 2H tothe 1T phase. This provides strong evidence that the improvedHER catalytic activity of the MoS2 nanowires after lithiation is aresult of the disorder caused by the intercalation, exposingmore active defect sites for HER catalysis, rather than a crystalphase transformation.XPS spectra of the nanowire samples are shown in Figure 3B.

The as-grown nanowires clearly show the trigonal prismatic2H-MoS2 shell on a reduced MoOx core, with prominent peaksat ∼229 and 232 eV, corresponding to Mo4+ 3d5/2 and Mo4+

3d3/2 of 2H-MoS2, respectively, denoted by the red curve. Thestrong peak at 232 eV is a convolution of the Mo4+ 3d3/2 fromMoS2 and the 3d5/2 of Mo6+ from the reduced molybdenumoxide (MoOx) core, along with its corresponding doublet 3d3/2at ∼236 eV, denoted by the blue curve. 2H-MoS2 has acharacteristic doublet peak resulting from the anion for S 2p3/2and S 2p1/2, which are seen at ∼162.5 and ∼163.7 eV,respectively. The ratio of the relative intensities of the S 2p andMo 3d peaks corresponding to 2H-MoS2 leads to a S:Mo ratioof ∼2:1, consistent with the expected elemental compositionfor MoS2.After lithiation of the nanowires, the ratio of XPS peak

intensities corresponding to 2H-MoS2 is not significantlyaltered, still maintaining a S:Mo ratio of ∼2:1, indicating thatthe MoS2 shell has not changed stoichiometry. Analysis of corelevel binding energies of sulfur 2p shows no change in energy.In the nanowire samples, there is a very weak sulfur peak at a

Figure 3. (A) Raman spectroscopy of MoS2 nanowire arrays before (black curve) and after lithiation (red curve), as well as chemically exfoliated 1T-MoS2 sheets (green curve) and 2H-MoS2 sheets (blue curve). The locations of the J2 and J3 for 1T-MoS2 are shown by the shaded area. (B) XPSspectra showing Mo 3d, S 2s, as well as the S 2p core level binding energy regions for MoS2 nanowire arrays both as grown and after chemicalexfoliation. Chemically exfoliated 1T-MoS2 sheets and 2H-MoS2 sheet analysis is shown for comparison.

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higher binding energy, which denotes mild oxidation on thesurface. XPS of the 1T octahedral phase MoS2, indicated by thepurple curve, shows a large (0.9 eV) shift of the Mo 3d bindingenergies and a corresponding shift and broadening of the S 2penergies.13,45,54 This is clearly not the case in the lithiation ofthe core−shell MoOx/MoS2 nanowire arrays, which maintainthe 2H-MoS2 phase after lithium intercalation, and anymodification is due to changes in morphology of the nanowires.Analysis of the Mo 3d binding energies of the nanowires afterexfoliation shows an additional oxidation state, which suggeststhat the intercalated lithium interacts with the MoOx core andstarts to reduce it further.A critical question still remains: why lithium intercalation of

MoS2 particles leads to a phase transition and formation of the1T metallic phase, whereas the core−shell MoOx/MoS2nanowires do not exhibit this phase transition followinglithiation. During lithium intercalation of bulk MoS2, lithiumdonates an electron to the crystal lattice, inducing a 60° strainleading to the phase transition from semiconducting 2H to themetastable metallic 1T phase.13,41,55 However, the molybde-num and sulfur binding energies composing the MoS2 shell ofthe core−shell nanowires are unaffected following exfoliation.The volume expansion during the intercalation leads todisruption of the nanowire shell, causing disorder and thecreation of more active defect and crystal edge sites, but the

original 2H-MoS2 crystal structure is maintained. To furtherobtain the mechanistic origin of catalytic activity in structures,temperature-dependent annealing studies were performed onboth nanowires and nanosheets.

3.5. Insight into the Mechanism via ControlledAnnealing Studies. We performed step-by-step annealing,and the subsequent electrochemical activity is measured toaddress the origin of catalytic activity. As shown in Figure 4A, alinear and gradual decrease in the current density is observedfor nanowires up to 250 °C, whereas a sharp drop in currentdensity is obtained for the exfoliated 2D sheets at 150 °C,indicating the phase transformation from metallic 1T tosemiconducting 2H.45 No such phenomenon is observed forthe annealed nanowires. A clearer indication is obtained fromTafel plots (Figure 4B); above 150 °C no change in the Tafelslope is observed for the exfoliated nanowires, which remainsconstant (64−65 mV/decade from 150 to 250 °C), indicatingthat the desorption of hydrogen remains the dominant rate-limiting step, and there is no change in catalytic sites duringannealing. However, the Tafel slope for the 2D sheets increasesfrom 54 to 70 mV/decade between annealing at 150 and 250°C, indicating that hydrogen adsorption (Volmer) emerges asthe rate-limiting step, and different sites are now contributingto the catalysis, further indicating a crystal transformation fromthe metastable 1T to 2H phase. The slight decrease in

Figure 4. (A) Current density (mA/cm2) at −0.3 mV vs RHE for nanowires (red curve) and nanosheets (black curve) as a function of annealingtemperature. Current density was normalized with current density value at 250 °C. (B) Comparison of Tafel slope of nanowires (red curve) andnanosheets (black curve) with increasing annealing temperatures. (C) Schematic showing the morphology effects with increasing temperature:crystal phase conversion in the 2D sheets and shift toward adsorption rate-limited kinetics, whereas agglomeration of the nanowires is the reason forloss of minimal activity, with desorption as the rate-limiting step.

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electrochemical activity in the exfoliated nanowires is possiblydue to agglomeration or sintering of the wires, leading todecreased surface area, but there is no change in the catalyticactive sites or the HER on-set potentials (Figure S7). Figure 4Cshows a cartoon demonstrating the effects of annealing on theMoS2 2D sheets compared to the lithium-intercalated nano-wires. This annealing experiment is further proof that thelithium intercalated MoOx/MoS2 core/shell nanowires did notundergo phase transition to form the 1T phase but also helpsdemonstrate the thermal stability of these nanowires.

4. CONCLUSIONHere, we report the enhancement of HER catalytic activity ofcore−shell MoOx/MoS2 nanowires following chemical exfolia-tion. The rapid volume expansion, which occurs due to lithiumintercalation, disrupts the ordered crystal structure, leading torandom orientations and the exposure of more catalyticallyactive defect sites, a combination of increased quantity andincreased activity of those defect sites. These defect sites aresupported on a high surface area, conductive reducedmolybdenum oxide nanowire, which results in improved chargetransfer characteristics. This leads to a 50 mV improvement inthe HER onset potential, as well as a 600% increase in thegenerated current density at −0.35 V vs RHE. Furthermore,Raman, XPS, and controlled annealing studies corroborate thatthe increased catalytic activity in the nanowires does not resultfrom the formation of the metastable 1T-MoS2 phase, which istypically formed by lithium exfoliation of bulk MoS2 particles,but from the exposure of higher catalytically active surface areaof the stable 2H-MoS2 phase. The decrease in Tafel slope afterlithiation experimentally confirms that more sites for hydrogenabsorption, i.e., edge and defect sites, are exposed during thedisruption of the MoS2 shell. The thermal stability of theexfoliated nanowires, compared to the 1T MoS2 sheetsfollowing annealing, is further confirmation of the differentmechanisms of catalysis in the two architectures. This is one ofthe first reports on the effects of intercalating lithium intocore−shell chalcogenide nanowires, as well as one of the firstinvestigations of the crystallographic effects of lithiumintercalation on 1D nanowire morphologies.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.jpcc.5b05640.

Schematic showing MoS2 core−shell nanowire synthesisscheme. XRD patterns of bare MoO3 nanowire array andMoO3/MoS2 core−shell nanowires. Electron energy lossspectra (EELS) of the lithium-intercalated MoS2 nano-wires. Details of crystallographic characterization techni-ques. Nyquist plot for series resistance in electrochemicaltesting. Electrocatalytic testing of bulk MoS2 powder.Results of incremental lithiation. Double-layer capaci-tance measurements of nanowires and 2D sheets. Effectsof annealing of nanowires and nanosheets (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: gautam@lanl.gov*E-mail: mahendra@louisville.edu.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

The authors would like to acknowledge Center for IntegratedNanotechnologies and LDRD Funding at Los Alamos NationalLaboratory, as well as the Conn Center for Renewable EnergyResearch at the University of Louisville for facilities and accessto characterization equipment. Development of samples andcharacterization was supported partially by DOE EPSCoR (DE-FG02-07ER46375) and by a graduate fellowship funded byNASA Kentucky under NASA award No: NNX10AL96H.

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DOI: 10.1021/acs.jpcc.5b05640J. Phys. Chem. C 2015, 119, 22908−22914

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